devel/libvirt: fix build on 15/aarch64

The build picked up BHYVE option while it does not have the
/usr/sbin/bhyveload executable which is needed by the build.

PR: 282786
Reported by: https://lists.freebsd.org/archives/freebsd-pkg-fallout/2024-December/690521.html
Approved by: novel@ (maintainer timeout)